|By David W. Spitzer, P.E.|
Process switches have been used for years in process plants for sensing flow, level, pressure, temperature, and position limits to name a few. Some switches are required by codes, such as when low and high-pressure switches are used to shut down a boiler when certain pressures are outside of safe limits. Many systems use valve limit switches to verify valve position according to the process control strategy.
Process instruments in general, and process switches in particular, can and do fail. When a process switch fails, it will typically fail to a condition that causes its contacts to remain in a fault or safe state. If the contacts indicate a fault state, actions may occur to shutdown the system and diagnose the problem. However, if the contacts remain in a safe state, the switch will not cause the actions necessary to react to a potentially dangerous condition. In short, the problem is that process switch failures can go undetected for extended periods, unless the switch is periodically tested. Periodic testing may be quite inconvenient and time consuming. In some processes, a process shutdown may be required. However, shutdown and startup operations that increase the risk of an adverse event, may adversely affect the feasibility of testing.
On the other hand, transmitters provide a continuous stream of information to assess its operational health by observation. The measurements can also be used to verify the operational health of other process measurement instruments. For example, a tank prone to overfilling may have a differential pressure level transmitter to control the tank level and a capacitance level switch that alarms when the tank level is high. Installing a capacitance level transmitter in place of the switch can provide an additional level measurement that verifies the proper operation of the differential pressure level transmitter. With the advent of fieldbus communications, transmitter health can be monitored continuously. This is in stark contrast to process switches where the operation is verified by testing the device, or when the level becomes excessively high.
Installing transmitters instead of process switches is generally more expensive. However, this need not be the case. In the previous example where one level is being sensed, the cost difference is the difference between the cost of the level switch and the level transmitter because the wiring, conduit, and other infrastructure items are needed for either instrument. Installations that segregate contact input/output wiring may result in savings due to the elimination of some of this infrastructure. If more than one level switch were needed, the installed cost of the transmitter would likely be lower than that of the level switches.
Design guidelines that can be used to eliminate valve limit switches are subtler. Valve limit switches are typically used to sense the position of the valve shaft as a surrogate to determine whether the valve is open or closed. In some applications, a flowmeter can eliminate the need for some valve limit switches by directly determining whether flow exists in the pipe. For example, in a truck loading system, the open limit switch on the final valve at the truck might not be necessary if the flow rate were measured. Similarly, it may be possible to eliminate other valve limit switches in the system after careful consideration and hazard analysis.
The design trend over the last decade appears to be away from process switches and towards the installation of transmitters. With careful design, instrumentation systems with more versatility that use transmitters instead of process switches can be installed at nearly the same or lower cost than instrumentation systems using process switches.
About the Author
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, start-up, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME MFC and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to… series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.
For More Information: www.spitzerandboyes.com